1. Technical Field
The present invention relates to a cleaning system control method, a fixing device, and an image forming apparatus incorporating the same, and more particularly, to a method for controlling a cleaning system that includes a cleaning web to clean a rotary member, a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus which employs a fixing device with a cleaning capability.
2. Background Art
Modern electrophotographic printers have become increasingly sophisticated to provide a high-speed, high-quality color imaging capability that can produce a clear, sharp image comparable to that of offset lithographic printing. In electrophotographic image forming apparatuses, such as printers, photocopiers, facsimile machines, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process may be followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium.
Various types of fixing equipment are known in the art, among which a pressure-assisted thermal fixing device is widely adopted. This type of fixing device employs a pair of generally cylindrical members, such as a looped belt and a roller, one having a heat source such as a halogen heater or the like for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image onto the medium under heat and pressure.
A particular type of such thermal fixing device is one that employs an endless fuser belt entrained around a fuser roller and a heat roller, with a pressure roller pressing against the fuser roller through the belt to form a fixing nip therebetween. The endless belt exhibits a relatively low heat capacity and thus can be heated more rapidly and sufficiently than a cylindrical roller. Also, compared to a roller-based assembly, the belt-based assembly features an increased size of the fuser roller and the pressure roller, leading to a longer fixing nip and a longer duration of nip dwell time during which a recording medium is subjected to heating within the fixing nip. Further, with suitable adjustment to the stiffness and wall thickness of the fuser roller, the belt-based assembly enables ready separation of recording media from the fuser roller.
With its superiority in terms of heating performance, nip dwell time, and media separation, the belt-based fixing device finds useful application in high-speed color printers, which require efficient heating of the fixing nip and good separation of recording media from the fuser member.
One problem associated with the pressure-assisted thermal fixing device is undesired transfer or offset of toner particles from a recording medium to a fixing member. Ideally, a toner image after fixing permanently adheres to a recording medium on which it is printed. However, toner offset often takes place, for example, due to improper heating at the fixing nip, where adhesion between the fuser member and the fused toner exceeds that between the recording medium and the fused toner, causing a small portion of toner to transfer from the recording medium to an adjoining fixing member.
Two types of toner offset are known: cold offset and hot offset. Cold offset occurs where insufficient heating at the fixing nip causes the toner image to fuse only superficially, leaving an inner portion of the toner layer in a loose, unfused state, which can partially crush up and eventually migrate to the fixing member. Such toner migration is typically accompanied by concomitant image defects, in which the toner image, which is not completely fused or fixed, easily rubs off the printed surface being output. Hot offset, on the other hand, occurs where excessive heating at the fixing nip affects viscoelasticity of the toner image being fused, so that the toner exhibits a high adhesion to the fuser member surpassing a cohesive force of toner particles, resulting in partial migration of toner to the fuser member.
To meet ever-increasing demands for high-quality imaging processes, current trends in electrophotographic printing are to formulate toner with extremely small particles or spherically shaped particles, as in the case of polymerized toner. The problem described above, in particular, cold offset, is pronounced where printing is performed using these newly developed types of toner. Compared to those with varying sizes and aspherical shapes, the small-sized, spherically-shaped toner is susceptible to causing cold offset since it does not easily conduct heat, and therefore is difficult to fuse and melt, particularly when used to print on a rough, irregular surface of uncoated paper.
Not surprisingly, toner offset detracts from image quality due not only to a lack of toner falling off from the recording medium, but also to soiling of the resulting print with offset toner which, once transferred from a recording medium onto the fixing member, is again transferred to another recording medium that enters the fixing nip subsequent to the foregoing recording medium.
Various cleaning techniques have been proposed to keep the fixing member clean of toner particles and other contaminants, which employ a cleaning web, such as an elongated strip of unwoven fabric, to wipe the surface of the fixing member. In a typical configuration, the cleaning web is drawn from a replaceable supply roller and pulled by and wound on a takeup roller, with a tension roller elastically biasing the web against the fixing member to form a cleaning nip therebetween, at which the web rubs against the fixing member.
Such a web-based cleaning system may be designed to constantly supply a new, unused portion of the cleaning web to the cleaning nip, so as to prevent formation of a gap between the fixing member and the web, which, if created, would permit small spherical toner particles to escape from being wiped off at the cleaning nip. Although capable of effectively cleaning the fixing member, such constant supply of cleaning web results in wasteful use of the cleaning web, which is detrimental to environment. Also, accelerated consumption of the cleaning web requires frequent service for the cleaning system and thus eventually increases maintenance cost of the image forming apparatus.
To counteract the problem, a control method has been proposed for a cleaning system which controls supply of a cleaning web from the supply roll into the cleaning nip. According to this method, the controller adjusts an amount by which the cleaning web is taken up depending on image density (i.e., a ratio of a toner-covered area to an entire image area) of a specific print job processed through the fixing nip. Although designed to prevent an unnecessary, superfluous supply of cleaning web upon processing of relatively light or low-density images, however, such control does not work properly because the image density is not always proportional to the amount of toner offset.
Still another technique has been proposed to provide a cleaning system with increased durability and functionality, which controls activation of a takeup roller to supply an unused portion of the cleaning web to the cleaning nip upon completion of a single print job. Although generally successful for its intended purposes, the technique fails to eliminate an unnecessary supply of the cleaning web, resulting in a shorter lifetime of the cleaning web than would be desired.